A TEST OF THE INTRINSIC NATURE OF THE SHALLOW PROTON TRAPS IN ICE

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A TEST OF THE INTRINSIC NATURE OF THE SHALLOW PROTON TRAPS IN ICE

P. Wooldridge, J. Devlin

To cite this version:

P. Wooldridge, J. Devlin. A TEST OF THE INTRINSIC NATURE OF THE SHALLOW PROTON TRAPS IN ICE. Journal de Physique Colloques, 1987, 48 (C1), pp.C1-641-C1-644.

�10.1051/jphyscol:1987189�. �jpa-00226455�

JOURNAL DE PHYSIQUE

C o l l o q u e C1, suppltiment a u n o 3, Tome

m a r s 1987

A TEST OF THE INTRINSIC NATURE OF THE SHALLOW PROTON TRAPS IN ICE(')

P . J .

WOOLDRIDGE a n d J . P . DEVLIN

D e p a r t m e n t o f C h e m i s t r y , Oklahoma S t a t e U n i v e r s i t y , S t i l l w a t e r ,

74078,

Abstract

was recently suggested by Warman and Kunst t h a t t h e slow secmdary decay of solvated electrons, generated i n i c e by pulsed-electron-beam radiolysis, is indicative of t h e presence of r e l a t i v e l y shallow proton t r a p s t h a t delays t h e --geminate recatbination of proton-solvated electron p a i r s (1). Their d e l for t h e k i n e t i c s of t h e decay of e-(sol), which extends i n t o the microseccnd range a t 270

invoked t h e presence of a pseudoequilibrium between m b i l e protons and p r o t o n s immobilized by a s s o c i a t i o n w i t h shallow t r a p s . The t r a p s were tentatively identified a s Bjerrum I r d e f e c t s which a r e known to have an associated p a r t i a l negative charge (2) . Additional strong evidence, f o r t h e importance of shallow proton t r a p s in ice following radiolysis, was subsequently cbtained from

Although irradiation with 1.7

electrons produced cnly limited proton exchange in cubic ice a t 90

t h e subsequent warming

the

into the 125

resulted in t h e rapid ccnversion of isolated D20 to neighbor coupled IED molecules.

Since t h e thermal generation of t h e w b i l e protons required f o r t h i s exchange reaction requires tenperatures i n excess of 135

and r e s u l t s i n the £omtion of isolated IED ( 4 ) , protons escaping from shallow t r a p s were c l e a r l y responsible f o r t h e 125

reaction.

S i n c e e l e c t r o n beam r a d i o l y s i s g e n e r a t e s numerous d e f e c t s i n ice and, when prolonged, can ccnvert c r y s t a l l i n e ice to m r p h o u s ice, the rather conclusive evidence t h a t protons a r e shallowly trapped i n ice following radiolysis is only suggestive of t h e existence of such t r a p s i n pure ice. On t h e other hand, the question of t h e i n t r i n s i c o r e x t r i n s i c nature of t h e t r a p s is of ccnsiderable ccnsequence i n t h e analysis of both k i n e t i c and thermodynamic praperties of im-pair defects i n ice. For these r e a m s e f f o r t s to evaluate t h e nature of t h e shallow t r a p s a r e warranted and one such e f f o r t , based on t h e photoionization of ice, is described.

Pure H20 i c e ccntaining isolated i n t a c t D20

2%) was g r o m a s polycrystalline thin f i l m s by e p i t a x i a l d e p o s i t i o n a t 125 K. FT-IR s p e c t r a , measured i n t h e O-D stretching region before and a f t e r i r r a d i a t i o n of t h e thin film w i t h a few watts from a mercury resonance lanp a t 90 K, indicated t h a t only a very minor extent of proton exchange was induced during two hours of uv-photolysis (curves

and

of Figure 1).

Hawever, as was reported for the electron-beam radiolysis experiments, rapid protrJn excharge was observed when t h e photolized samples were warrned i n t o the 120

range.

The positiops and a s s i g m n t s of t h e

s t r e t c h i n g d e infrared bands for D@,

and neighbor-oupled IED, i.e.

2, have been thoroughly examined i n an e a r l i e r study (4). Based cn these band a s s i g m n t s it is c l e a r from curve C of Fig. 1 t h a t the isotapic exchange that occurred a t 120

which is el1 below the tenperature a t which exchange is i n i t i a t e d thermally £or pure ice (i.e., 135 K ) , did not produce

isolated

t h e usual t h e m 1 product, but rather neighbor coupled

u n i t s (i.e., (HOD) 2) . I n f a c t , t h e increase in intensity of t h e 2422 cm-I band of isolated

was so s l i g h t t h a t t h e isolated HDD band has been e f f e c t i v e l y remved from

the curves i n Fig. 1 by subtraction of t h e absorbtion i n t e n s i t y caused by t h e

(')~irst published in J. Chern. Phys.. 84, 4111 (1984)

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987189

Cl-642

DE

i n i t i a l amunt of cantaminant isolated

By cantrast t h e (HOD)2 doublet a t 2400 and 2445 cm-1 (labeled by crosses i n Fig. 1) appeared and became intense a s a r e s u l t

of protan transfer within- the photbianized i c e annealed a t 120 K.

These data indicate t h a t uv photolysis r e s u l t s in the photoionization of ice but t h e protons produced a r e very rapidly h b i l i z e d a t 90

and, therefore, cause very little proton exchange. The subsequent warming of t h e photolyzed sanples to 120

w b i l i z e s a few of the induced proton defects a t a temperature a t which the thermal ionization of ice is t r i v i a l i n extent and any orientational defects present a r e h b i l e . Consequently, the photoionization leads to proton happing i n the absence o f t h e turn-step of t h e h o p t u r n mechanism o f proton t r a n s p o r t .

necessary cansequence of t h e hop-step, in t h e absence of t h e turn-step, is t h e nearly exclusive formation of (HOD12 from isolated D@ a s has previously been described f o r i c e annealed following electron b o n h r b n t (3).

x - D,O T : l 2 0 K

+ - (HOD)*

These r e s u l t s show t h a t photoionization of ice with w l i g h t produces protons t h a t a r e shallowly trapped and therefore released a t a r e l a t i v e l y l o w temperature.

However, since uv photolysis of ice is

to produce a variety of products

HS2) ( 5 ) , t h i s r e s u l t does not establish t h a t the presence of t h e shallow t r a p s

is an i n t r i n s i c property of pure ice. However, the damage to t h e ice c r y s t a l s is minor ccmpared

t h a t induced by electron b u b a r h n t so t h e detection of these

traps i n the present study greatly strengthens t h a t possibility.

--

Support of this research by t h e National Science Foudation under Grant

8420961 is g r a t e f u l l y acknowledged.

Figure 1

Infrared spectra f o r cubic ice containing isolated D20. Curves A and B a r e

-

before and a f t e r 2h of photolysis a t 90K.

While C and

a r e following annealing of photolized sample f o r 3 h and 24 hours a t 120

Curve

is the r a t i o of curves

and

An absorbance, c o r r e s p o n d i n g t o t h e I I

i n i t i a l amxlnt of contaminant isolated

2500 2 4 0 0 2300

- 1

has been substracted i n each case.

cm

References

(1)'Kunst

and Warman

Phys. Chem ,87 (1983), 1093.

( 2 )

Scheiner

and Nagle J.F.,

Phys. Chem.,87 (1983), 4267.

(3) Devlin J.P. and Richardson

J.Chem.Phys. ,81, (1984) , 3250.

( 4 ) C o l l i e r W.D. Ritzhaupt

and Devlin J.P.,J.Phys.Chem.,88, (1981),363.

(5) G h o d e y

and Hochanadel C.J., J.Phys.Chem.,75, (1971),40.

COMMENTS

J.W. GLEN

1 - Would not proton injecting electrodes of the kind described by Petrenko and others (1983) be better for your purposes than the methods you describe, since they should inject protons with less damage

2 - Is there not also the possibility with only H30+ defects of moving both deuteron of the D20 oxygen so giving two HOD separated by one (but only one) H20 molecule

V.F. Petrenko, R.W. Whitworth and J.W. Glen , Phil. Mag. B, 47, no 3, (19831, 259-278.

Answers

1.

The possibility that the H+ concentration and thereby the hopping rates could be manipulated independently of the L-defect migration rates in this manner was called to my attention about six years ago by Mark Sceats, but Mark, who is in the audience, indicates that he has not tried the experiment. Neither have we.

2. You are right, but the equilibrium concentration of such a species (in a system for which hopping but not turning is possible) is, based on statistics plus relative proton and deuteron hope rates, less than 10

of the (HOD)2 value and not spectroscopically determined.

G. W. GROSS

The existence of shallow electron and/or proton traps is of great interest for the study of charge transfer processes between ice particles in clouds at temperatures around 250°K (much higher than the experiments reported in this paper). This begs the question of Che possible relevance that these results might have for the existence and nature of surface proton and electron states in ice.

Answer

We are working with molecular probes distributed uniformly through the bulk. For t h i s and other r e a s o n s we b e l i e v e (and hope) t h a t we a r e e x a m i n i n g more-surface-determined characteristics. However, as described by Dr. Plummer earlier today the L-defects and therefore the traps, may also exist at surfaces.

P.L.M. PLUMMER

With respect to surface defects and trapping of H+, our previous semi-empirical quantum mechanical calculations (Cambridge Conference) found protons strongly attracted to L-defects. In preliminary ab initio studies now underway, find the same attraction so I would expect that the surface defects could be effective proton traps in naturally occuring ice.

Answer

It might be relevant to note that, for the internal shallow traps, Warman has

reported a trap depth of rv 12 kcal and we find a value of

10.5 kcal based on the

T-dependence of the ratio of (HOD)2 formation produced by protons escaping from the

traps.

JOURNAL

DE

J.M.

With regard t o whether r a d i a t i o n produced protons a r e trapped a t o t h e r r a d i a t i o n produced c e n t r e s o r a t i n t r i n s i c d e f e c t s i n t h e l a t t i c e

on studying t h e decay of mobile protons subsequent t o nanosecond pulsed i r r a d i a t i o n of i c e we do not observe

decrease i n l i f e t i m e with i n c r e a s i n g dose i n t h e pulse. This may be taken t o show t h a t l o c a l i s a t i o n

occuring by t r a p p i n g a t an i n t r i n s i c d e f e c t . These experiments however were c a r r i e d o u t above

where t h e e q u i l i b r i u m c o n c e n t r a t i o n of i n t r i n s i c d e f e c t s would be expected t o be much higher than f o r t h e 90°K r a d i o l y s i s c o n d i t i o n s o f your work.

Answer

I

did not mean t h a t we have any r e a l reason t o b e l i e v e t h a t r a d i a t i o n damage

t h e

cause o f t h e shallow H+ t r a p p i n g observed by Warman, but only t o emphasize t h a t , i n

g e n e r a l , such damage does o c c u r and can be e x t e n s i v e (Obviously, our s t u d y is

s t r o n g l y based on Warman1s o b s e r v a t i o n s and a t t h i s t i m e we c o n c u r w i t h h i s

important r e s u l t ) .